Recently, environmental consciousness has been increased by a global warming phenomenon and a novel energy system generating no greenhouse gases such as carbon dioxide gas has attracted attention. Environment-friendly recyclable energy such as photovoltaic power generation and wind power generation does not emit gases, which are said to induce global heating, such as carbon dioxide gas, it has been actively studied as clean energy. A solar cell and solar thermal power generation have attracted attention because of excellent safety and easy-to-handle.
As a typical solar thermal power generation method, there are a central system (central tower system), a distributed system (parabolic trough) and a dish/stirling system. In these systems, sunlight is collected to a spot by use of a reflecting mirror and heat of sunlight harvested is converted into electric energy through thermoelectric conversion. While the key point of this system is collecting light without loss, as the factor significantly varying efficiency, a reduction of reflectivity caused by stain of a reflecting mirror is particularly concerned as a problem. Furthermore, a solar cell whose light-receiving surface is protected by a protecting cover formed of glass, a weather-resistant resin film and the like, has an analogous problem of reducing the light transmittance and thus an energy conversion efficiency of the solar cell since the protecting cover is stained with smoke dust during long-term use.
As a technique for preventing surface stain, for example, Patent Literature 1 discloses a technology for forming a surface layer by applying a coating liquid, which contains tungstic acid dissolved in ammonia water and distilled water, onto a layer containing an anatase type titanium oxide, and then baking at 700° C. to obtain a layer formed of tungsten oxide. Furthermore, an attempt to improve antifouling properties and the like by blending not only an inorganic component but also an organic component has been made.